Having an effective and reliable strategy for fire safety is of the utmost importance, particularly in isolated and enclosed environments, such as in terrestrial vehicles and aircraft, and partial-gravity conditions, such as in spacecraft and extraterrestrial manned enclosures. For example, the National Aeronautics and Space Administration (NASA) uses carbon dioxide (CO2) for fire suppression on the International Space Station (ISS) and halon chemical extinguishers on the Space Shuttle.
While each of these technologies is effective, they also have drawbacks.
The toxicity of carbon dioxide (threshold limit value (TLV)=5000 ppm) requires that the crew wear breathing apparatus when the extinguishers are deployed. Furthermore, the subsequent removal of the discharge CO2 will tax the spacecraft's Environmental Control and Life Support System (ECLSS).
Halon use in future spacecraft has been taken out of consideration by NASA out of observance of the international protocols against substances that destroy the ozone layer. Gaseous agents used in halon fire-fighting systems have been associated with depletion of the ozone layer, and their use is being phased out around the world. A timetable for replacement was developed as part of the Montreal Protocol, which has encouraged a significant effort here and abroad to identify replacement agents that are as effective as halons, but do not impact the environment. To date, this effort has focused on near-term substitution of other halocarbon compounds, including halochlorofluorocarbons (HCFCs), halofluorocarbons (HFCs) and perfluorocarbons. Although halon deployed in low earth orbit (LEO) or farther out will not come into contact with the Earth's ozone layer, NASA protocols require de-orbiting of a spacecraft after deployment of a halon extinguisher because the ECLSS systems have no means of scrubbing bromofluorocarbons. Another issue is the loss of fire protection once the halon system has been discharged.
An important area of research on halon replacements has been in the use of fine water mists for fire suppression. Fine water mist can suppress fires by attacking all three legs of the “fire triangle”: heat, radiation, and fuel source. Water mist can take away heat from the fire as both sensible and latent heat. Perhaps surprisingly, research has shown that the sensible heat effects of water are as significant as the latent heat. However, the heat of vaporization is still important in removing energy from the fire. The steam produced can then act as an inerting agent, or diluent, to inhibit fire propagation. Finally, water mist can act to wet surfaces, which reduces the volatilization of solids and thus the amount of fuel present. An additional mechanism by which water mist can inhibit fires is through the attenuation of infrared radiation. A water aerosol becomes an optically dense medium that prevents the infrared heating of unburned surfaces by burning surfaces. Also, the nitrogen gas used in the generation and propulsion of the fine water mist displaces the oxygen, thereby removing a combustion component from the fire.
Fine water mists hold considerable promise as fire suppression agents. Important design criterion for fine water mist extinguishers include the droplet properties of size and momentum, which are in large part controlled by the atomizer/nozzle design. Engineering of fine mist systems for specific applications is needed, because development of fine mist technology is in an early stage.